This invention relates to ultrasonic diagnostic systems which measure the velocity of blood flow using spectral Doppler techniques. In particular, the invention relates to the optimal presentation of such information in a continuous time-line display.
Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers, such as blood cells. This frequency shift translates into the velocity of the blood flow.
In state-of-the-art ultrasonic scanners, the pulsed or continuous wave (CW) Doppler waveform is computed and displayed by a video processor in real-time as a gray-scale spectrogram of velocity versus time with the gray-scale intensity (or color) modulated by the spectral power. The data for each spectral line comprises a multiplicity of frequency data bins for different frequency intervals, the spectral power data in each bin for a respective spectral line being displayed in a respective pixel of a respective column of pixels on the display monitor. Each spectral line represents an instantaneous measurement of blood flow.
Two factors which affect the display contrast resolution of the Doppler spectrogram are the data compression dynamic range (DR) and the video processor's gray map. Both of these factors typically are user-adjustable via front panel control keys. A logarithmic function is usually used to compress the FFT spectra resulting from ultrasound echo signals so they can be displayed on a video monitor which typically has a gray scale resolution limited to 6-8 bits or 64 to 256 gray levels. FIG. 2A illustrates some typical display DR settings for log compression. The compressed data (y) is further transformed to gray levels (z) via a gray map (FIG. 2B). A computer host typically specifies which compression function (FIG. 2A) and gray map are to be used based on a user's input via front-panel control keys. In practice, as shown in FIG. 2A, the y versus 10log(x) compression map is often pivoted at some intermediate y value in an attempt to keep the typical signal level at a constant video intensity for the different dynamic range settings. This means that for very small or very large gain settings, the probability density distribution may saturate at the low or high end of the dynamic range (typically 20-40 dB).
For best visualization and analysis (waveform tracing) of the spectrogram, the display DR (FIG. 2A) should be chosen based on the spectral signal DR (range of spectral amplitudes). In practice, however, the latter is not readily available to the user, and DR and gray map adjustments are seldom made. Instead, the pre-set DR and gray map for a given application type is used. The shortcoming of the preset approach is that for the same application type, the Doppler signal DR may vary by more than 10 dB between different vessels or different subjects. Any manual DR adjustment based on visual judgment is subjective and often requires a corresponding system gain change because the mean signal level may not be at the pivot point (FIG. 2A) of the compression curves.
Some form of automatic gain control may be used to ensure that the spectral noise floor stays constant as a function of depth, and/or to turn down the system gain automatically if saturation is detected. However, it should be noted that such gain control does not address what DR and gray map should be used in order to display the spectrogram with optimal perceived contrast.